1. Field of Invention
The invention relates to improvements to a linear electrical machine for electric power generation or motive drive. In some variations, the invention relates to a free piston engine and alternator in combination. In some further variations, the invention relates to mechanisms for providing restoring forces to pistons in such engines, for example in free piston Stirling engines.
2. Discussion of Related Art
Quiet and efficient electric power generation can be important in a variety of applications. For example, boats and other spaces having power generation systems in close proximity to people have a need for quiet operation. As a result, turbines, internal combustion engines and other power sources are often far too noisy for use in such applications. Free piston Stirling engines, however, operate fairly quietly and have been used to drive linear electrical machines also referred to as linear alternators to generate electric power. Except as otherwise necessitated by context, the term “alternator” is used herein to generically refer to any type of electric power generation device, whether producing alternating current, direct current, or other forms of electric power. Except for the case of the automotive “alternator” which has a built in rectifier to provide 12 volt DC output, the term “alternator” would otherwise be understood to be an electrical machine which produces AC power. These power generation systems are typically best suited by a linear alternator that can operate efficiently within the range of motion of a piston in the free piston Stirling engine (FPSE) that drives the alternator.
A conventional engine-alternator system produces a useful energy output in the form of electrical energy as a result of converting energy from one form to another, more useful form. In the case of a reciprocating system, the linear alternator converts the mechanical energy output by a reciprocating element of an engine into useful electrical energy. A conventional, FPSE has a harmonically reciprocating piston suitable for driving or carrying the moving component of the linear alternator.
In a conventional, FPSE, energy may be input by converting the chemical energy contained in a fuel into heat energy, or heat energy may be input from some other source. The engine converts heat energy into the mechanical energy of motion of a harmonically reciprocating power piston. Because the power piston reciprocates, a stroke in one direction has a beginning and an end, followed by a stroke in the opposite direction which returns the power piston to the beginning of the preceding stroke. A quantum of energy is expended to slow the power piston to a stop at the end of each stroke, after which the piston is caused to return to the beginning of that stroke. In conventional systems, the quantum of energy required may be stored in a spring or other mechanical device, or may be extracted from the useful electrical energy produced by the linear alternator. Such methods reduce the overall efficiency of the machine because of the late stage of energy conversion at which they are employed, and further because of the inefficient nature of the storage and retrieval mechanisms by which such quantum of energy is made available for such use.